The epidermal growth factor receptor (EGFR, HER1, erbBl), HER2/neu (erbB2), HER3 (erbB3), and HER4 (erbB4) comprise a family of homologous and interactive transmembrane receptor tyrosine kinases. Abundant evidence supports the role of this signaling network in mammary gland development, transformation, and tumor progression. Preclinical data suggest that the simultaneous blockade of the HER2 and EGFR tyrosine kinases exhibits a synergistic antitumor effect. We have hypothesized 1) that the simultaneous blockade of HER2 and EGFR in breast cancers will result in a measurable in situ antitumor effect in patients with operable breast cancer, 2) that the inhibition of the HER signaling network may have a greater antitumor effect on the natural history of breast cancers if used in untreated early disease and/or in patients with low tumor burden. The overall goals of this translational project are: first, to determine a molecular profile in untreated, operable breast cancer that will identify candidate patients for future adjuvant therapy trials with HER (erbB) signaling inhibitors; second, to identify novel surrogate markers of drug-induced inactivation of HER signaling that will shed light on mechanisms of action or resistance; and third, by imaging mass spectrometry, explore approaches to decipher the protein markers predictive of an antitumor response to HER signaling inhibitors. To achieve these goals, we propose the following specific aims: Specific Aim 1. To determine the anti-tumor effect of the HER signaling inhibitor OSI-774 in patients with untreated operable breast cancer and determine a molecular profile associated with evidence of response in situ. Specific Aim 2. To determine if OSI-774 inhibits EGFR and HER2 signaling in situ and whether drug-induced changes in phospho-HER3 and of phosphorylated p21 and p27 are surrogate markers of drug action. Specific Aim 3. To determine by imaging mass spectrometry the protein profiles in pre-and post-OSI-774 tumor specimens that predict for evidence of response in situ. Specific Aim 4. To determine if a threshold level of p27 is required for the anti-tumor action of HER signaling inhibitors like OSI-774.